The invention is related generally to lead screw drive mechanisms for medical infusion pumps, and more particularly, to a bearing system for mounting a lead screw in a syringe pump.
The infusion of fluids such as parenteral fluids into the human body is accomplished in many cases by means of a syringe pump having a lead screw and a screw drive device comprising a mechanism that translates the rotational motion of the lead screw into linear motion. The screw drive device is mounted to a syringe plunger driver head that typically grasps the plunger flange of a syringe and that applies the linear motion of the screw drive device to the syringe plunger to empty the syringe at a controlled rate.
Because syringes are of different lengths and are filled with different amounts of infusion fluids, the extension of the syringe plunger from the syringe barrel can differ from syringe to syringe. Many screw drive devices therefore include a disengagement mechanism that the operator uses to disengage the lead screw drive device from the lead screw threads. A disengagement mechanism control is typically located at the plunger driver head and can take the form of a lever or levers. Once disengaged, the operator may move the plunger driver head, and therefore the screw drive device to the position of the newly-mounted syringe plunger flange. The plunger driver head may then engage the syringe plunger flange and once engaged, the disengagement control may be released at which time the plunger driver head will grasp the plunger flange and the lead screw drive device will engage the threads of the lead screw at the new position. It is desirable that this disengagement mechanism and this plunger driver head be easy to use to facilitate operator usage of the pump.
Such a lead screw drive device with its integrated disengagement control and connected plunger driver head, although necessary, can impart substantial forces on almost any part of a lead screw. The screw drive device may be located at any position along the lead screw depending on the length of the syringe mounted for use and depending on the level of medical fluid remaining in the syringe. Additionally, certain medical fluids are more difficult to pump due to their viscosity or for other reasons, further placing an increased load on the lead screw. For these reasons, it is desirable to provide substantial mounting stability to the lead screw so that efficiency is maintained in the development of rotational movement, in the translation of that rotational movement to linear movement, and in the application of that linear movement to the syringe plunger head.
It is also the goal of syringe pump manufacturers to produce pumps having increased flow uniformity. That is, manufacturers strive to produce pumps that will pump exactly the selected flow rate throughout the infusion and not vary from that selected flow rate, until the syringe is exhausted or the rate is changed by the operator. However, mechanical tolerances of the syringe pump parts, interactions with the syringe, or other reasons can cause the flow rate of a syringe pump to vary from the selected rate. A variance from the prescribed and selected flow rate can be undesirable, especially if significant, in that the patient may not receive the desired level of the infusion fluid when needed. Manufacturers continue to refine their pump designs to reduce these variances in flow rate as much as possible.
In one lead screw arrangement, one end of the lead screw, i.e., a first end, is mounted through a transfer plate and has a pulley mounted to its end. The transfer plate forms a part of the inner frame of the syringe pump and consequently provides a stable and rigid mounting point for the lead screw. The lead screw pulley is directly engaged to the drive pulley of a motor through a drive belt. A bearing may surround the lead screw at the portion located through the transfer plate to lessen the effects of friction. In another arrangement, both the first end of the lead screw and the drive shaft of the motor may have gears and may be interconnected through an intermediate gear or gears, although this arrangement can result in less efficiency. In one design, the second end of the lead screw may also be mounted to a rigid plate with a bearing thus providing firm mounting to both ends of the lead screw. However, mounting the second end of the lead screw to a rigid mounting plate is not always an available option, especially when an extension tube must be used between the screw drive device and the plunger driver head.
The disengagement mechanism is typically formed as part of the drive device and permits selective engagement and disengagement of the drive device with the lead screw so that the drive device may be selectively positioned on the lead screw to accommodate different lengths of the syringes. A typical disengagement mechanism includes half-nuts that are spring loaded into contact with the threads of the lead screw. Through a series of levers and cams, the half-nuts may be moved outwards from engagement with the lead screw threads so that the drive device may be slid along the lead screw to the desired position. The length of the lead screw and the disengagement mechanism are designed to easily move the drive device along a substantial portion of the lead screw so that the smallest syringes and the largest syringes for which the pump is designed can be used with the pump.
In one particular design, the second end of the lead screw is located within a hollow connection tube that connects the screw drive device with the plunger driver head. The second end of the lead screw is not rigidly mounted but instead “floats” within the connection tube. The length of the lead screw is selected to exceed the travel of the syringe plunger within the syringe barrel so that syringes of various sizes may be accommodated. When the syringe barrel is full, the syringe plunger will be at the proximal end of the barrel with the plunger stem extended almost its entire length outside the syringe barrel. This configuration results in the overall syringe being almost twice the length of its barrel. Because some syringes are relatively long, the lead screw may be located at one end of the pump housing, for example the distal end, with the connection tube extending from the lead screw to a point near the other end of the housing, for example the proximal end, to engage the syringe plunger stem flange. However, the second end of the lead screw will always be located within the connection tube regardless of where the syringe plunger driver head is located.
In the approach described above where the second end of the lead screw is located within the hollow connection tube and is allowed to “float” in the tube, rigid mounting of that second end is not possible. Because there is a size difference between the outer diameter of the lead screw second end and the inner diameter of the hollow interior of the connection tube, the angle of the lead screw within the connection tube can change. Even a slight change in the angle between the two has been found to lessen the flow uniformity of the pump. The second lead screw end tends to move within the connection tube depending on the forces exerted on the lead screw thus adding inefficiency to the translation of the rotational motion of the lead screw to the linear motion of the screw drive device. The lead screw threads can change their angle of engagement with the screw drive device threads resulting in greater or lesser friction between the two and consequently resulting in lowered flow uniformity or flow accuracy of the pump.
A further undesirable effect of the floating second end of the lead screw is that it interacts with the interior of the connection tube scoring or gouging out the tube thereby imparting increased wear, and causing a larger difference in size between the lead screw and the connection tube thereby allowing for even more movement of the second end of the lead screw in the future.
In an effort to reduce the undesirable effects caused by movement of the floating lead screw, the floating second end of the lead screw has been hollowed to reduce its weight. This has been found to lessen the damage it does to the connection tube and can lower the amount of movement of the second end resulting in greater flow uniformity. However, the manufacturing process of hollowing a lead screw increases the cost of the screw as well as increases the rate of lead screw waste due to errors made during the hollowing process. This waste also increases manufacturing costs.
Hence, those skilled in the art have recognized a need for a stabilizing mechanism to be used with the second end of the lead screw so that the end is held in axial alignment with the connection tube and the drive device during operation. Further, those skilled in the art have recognized a need for reducing the costs of manufacturing a lead screw. The invention satisfies these needs and others.